英语“我正在拉大便”怎么翻译

说的俗一点呢，一般这样说
I'm shitting.
I'm craping.
I'm dropping shit/crap.

说的好听一点，礼貌一点，就不这么说了，含蓄点
I am in the toilet/loo/bathroom. 在上厕所
I am in the middle of something. 在忙

I was widening stool

I was constipated

满意了吧？！

I 'm making pupu